Still having this issue as well. I can sometimes download assets, but then soon after the app will turn to a grey screen and I will have to close and reopen it. And the desktop version of Quixel is only able to push assets to 4.27 and below. Very frustrating.
Edit: I think the issue is that I have a dual monitor setup with a 1080p (secondary) and 4k monitor (main) and there is some kinda bug inside of Quixel that does not like that. Because when I use quixel on my my 1080p monitor it works fine (at least for scrolling/browsing, still crashes sometimes).